An air compressor including a compressor body that compresses air, an electric motor for a power source for the compressor body, a gear mechanism that transmits power of the electric motor to the compressor body, and a gear casing that is connected between a casing of the compressor body and a casing of the electric motor and houses, e.g., the gear mechanism is known (for example, JP-A No. 2000-97186). The air compressor has a partitioning wall (part of a gear casing in JP-A No. 2000-97186) that defines the internal space of the gear casing and the internal space of the casing of the electric motor. A non-contact type seal is provided in a gap in a portion where a rotating shaft of the electric motor penetrates through the partitioning wall.
The seal has a threaded inner circumferential surface to generate a flow from the electric motor casing toward the gear casing when a rotating shaft of the electric motor rotates. This prevents lubricant in the gear casing from intruding into the casing of the electric motor.